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Introduction

Hydrothermal circulation across upper mantle rocks at mid-ocean ridges promotes serpentinisation reactions. In the course of these reactions, olivine reacts with water to form serpentine, magnetite and ferroan brucite, (Mg,Fe)(OH)₂, along with abiotic hydrogen (Moody, 1976

Moody, J.B. (1976) Serpentinization: a review. Lithos 9, 125–138. https://doi.org/10.1016/0024-4937(76)90030-X

). As observed in ophiolites (Neal and Stanger, 1983

Neal, C., Stanger, G. (1983) Hydrogen generation from mantle source rocks in Oman. Earth and Planetary Science Letters 66, 315–320. https://doi.org/10.1016/0012-821X(83)90144-9

; Abrajano et al., 1990

Abrajano, T.A., Sturchio, N.C., Kennedy, B.M., Lyon, G.L., Muehlenbachs, K., Bohlke, J.K. (1990) Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines. Applied Geochemistry 5, 625–630. https://doi.org/10.1016/0883-2927(90)90060-I

; Leong et al., 2023

Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H₂ and CH₄ outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008

), ultramafic rocks can still produce H₂ at low temperature (i.e. at T < 423 K), even if they are extensively serpentinised. The extrapolation of experimental kinetic data collected in the 473–623 K range (e.g., McCollom et al., 2016

McCollom, T.M., Klein, F., Robbins, M., Moskowitz, B., Berquó, T.S., Jöns, N., Bach, W., Templeton, A. (2016) Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine. Geochimica et Cosmochimica Acta 181, 175–200. https://doi.org/10.1016/j.gca.2016.03.002

) indicates that serpentinisation of olivine with a grain size of 500 μm should reach a reaction progress above 90 % in at least 10,000 yr at temperatures below 423 K.

The serpentinisation of olivine produces secondary minerals, including ferroan brucite, which are Fe²⁺-rich and which can further react to produce H₂ + magnetite at low temperature:

where [Fe(OH)₂]^brucite represents the Fe component of ferroan brucite.

Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017

Jöns, N., Kahl, W.-A., Bach, W. (2017) Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography. Lithos 268–271, 274–284. https://doi.org/10.1016/j.lithos.2016.11.014

; Klein et al., 2020

Klein, F., Humphris, S.E., Bach, W. (2020) Brucite formation and dissolution in oceanic serpentinite. Geochemical Perspectives Letters 16, 1–5. https://doi.org/10.7185/geochemlet.2035

; Ellison et al., 2021

Ellison, E.T., Templeton, A.S., Zeigler, S.D., Mayhew, L.E., Kelemen, P.B., Matter, J.M., The Oman Drilling Project Science Party (2021) Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite. Journal of Geophysical Research: Solid Earth 126, e2021JB021981. https://doi.org/10.1029/2021JB021981

) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H₂ production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017

Miller, H.M., Mayhew, L.E., Ellison, E.T., Kelemen, P., Kubo, M., Templeton, A.S. (2017) Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite. Geochimica et Cosmochimica Acta 209, 161–183. https://doi.org/10.1016/j.gca.2017.04.022

).

In order to test the potential of Reaction 1 to produce H₂ at temperatures below 423 K in ultramafic rocks, the kinetics and thermodynamics of Reaction 1 were investigated experimentally here using synthetic (Mg_1−x,Fe_x)(OH)₂ of grain size (40–100 nm) and composition (x from 0.156 to 0.205) relevant to natural ferroan brucite (Malvoisin et al., 2020

Malvoisin, B., Zhang, C., Müntener, O., Baumgartner, L.P., Kelemen, P.B., Oman Drilling Project Science Party (2020) Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project. Journal of Geophysical Research: Solid Earth 125, e2019JB018877. https://doi.org/10.1029/2019JB018877

).

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Materials and Methods

Ferroan brucite (Mg_1−x,Fe_x)(OH)₂, with x ranging from 0.156 to 0.205, was synthesised under ambient conditions from a stoichiometric solution of dissolved Fe(II) and Mg chlorides, as described in Carlin et al. (2023)

Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) Fe^III-substituted brucite: Hydrothermal synthesis from (Mg_0.8Fe^II_0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845

. The ferroan brucite obtained by this method formed platelets 40 to 100 nm across (Fig. 1a). It was loaded under an Ar atmosphere with degassed ultrapure water either in welded shut gold capsules (‘caps’ experiments) or in 50 mL Parr 5500 series titanium reactors (‘SP’ experiments; see details in Tables S-2 and S-3). The capsules were run either in horizontal cold seal pressure vessels at temperatures from 348 to 573 K at 20 MPa (caps#1 to #15) or in an oven at temperatures of 378 and 423 K at the liquid-vapour equilibrium pressure (P_sat, caps#t1 to #t12). Titanium reactor experiments, SP#3 to SP#5, were conducted in the 423 to 473 K range, also at P_sat. SP#6 was run at 378 K with an initial Ar pressure of ∼5 MPa. Pressure evolution in SP experiments was monitored at ±2 kPa with a Keller pressure sensor PA-33X. After the experiments, the H₂ produced and trapped in the gold capsule was sampled using the protocol described in Malvoisin et al. (2013)

Malvoisin, B., Brunet, F., Carlut, J., Montes-Hernandez, G., Findling, N., Lanson, M., Vidal, O., Bottero, J.-Y., Goffé, B. (2013) High-purity hydrogen gas from the reaction between BOF steel slag and water in the 473–673 K range. International Journal of Hydrogen Energy 38, 7382–7393. https://doi.org/10.1016/j.ijhydene.2013.03.163

and analysed by gas chromatography. H₂ leakage through the gold capsule walls is negligible at the temperatures investigated here (Malvoisin et al., 2013

Malvoisin, B., Brunet, F., Carlut, J., Montes-Hernandez, G., Findling, N., Lanson, M., Vidal, O., Bottero, J.-Y., Goffé, B. (2013) High-purity hydrogen gas from the reaction between BOF steel slag and water in the 473–673 K range. International Journal of Hydrogen Energy 38, 7382–7393. https://doi.org/10.1016/j.ijhydene.2013.03.163

). The amount of H₂ produced in titanium reactors was quantified either on the gas sampled at the run conditions, and/or at the end of the experiment. No H₂ was detected in two blank experiments performed at 378 and 473 K over more than 40 days with capsules containing only degassed ultrapure water (100 μL). Before experiments, the titanium reactors were heated to 523 K during one day in air to extract any H₂ potentially solubilised in the reactor wall (Louthan and Derrick, 1975

Louthan Jr., M.R., Derrick, R.G. (1975) Hydrogen transport in austenitic stainless steel. Corrosion Science 15, 565–577. https://doi.org/10.1016/0010-938X(75)90022-0

), and to ensure Ti surface oxidation prior to reaction. No hydrogen production was detected in these blank experiments. Details of the sample characterisation techniques are provided in the Supplementary Information.

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Results and Discussion

Ferroan brucite oxidation reaction. In all experiments, ferroan brucite partly decomposed into magnetite + H₂ (Table S-2). The H₂ yield increased from 4 × 10⁻³ to 220 μmol of H₂ per gram of starting material when the temperature increased from 348 to 573 K. In eight experiments (Table S-2), pyroaurite was detected (<20 wt. %), indicating the presence of minor CO₂ in the reacting medium. Pyroaurite does not involve significant H₂ production (Carlin et al., 2023

Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) Fe^III-substituted brucite: Hydrothermal synthesis from (Mg_0.8Fe^II_0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845

). Its impact on H₂ production rate is, thus, mainly associated with the lowering of the amount of ferroan brucite available for H₂ production. However, for experiments used for determining kinetic and thermodynamic parameters, we estimated that a maximum of 3.4 wt. % of the initial ferroan brucite was consumed to form pyroaurite. This leads to an error on the H₂ production rate that is small compared to the error associated with H₂ measurement of ∼11 %. The Fe content (X_Fe(OH)2) of ferroan brucite was determined from the refined unit cell parameters (Table S-2) based on Vegard’s law (Carlin et al., 2023

Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) Fe^III-substituted brucite: Hydrothermal synthesis from (Mg_0.8Fe^II_0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845

). It was found to vary from 0.1 to 0.2, i.e. equal or slightly below X_Fe(OH)2 in the ferroan brucite starting material. Recrystallisation of ferroan brucite as platelets was observed in the highest temperature experiments (Fig. 1). Unless metastable growth occurred, this observation suggests that ferroan brucite with X_Fe(OH)2 < 0.2 is a stable reaction product in these experiments.

Altogether, the experimental results revealed that brucite partly reacted during the experiments according to Reaction 1, leading to the overall reaction:

where x is the initial X_Fe(OH)2 and y is a parameter related to reaction progress such as 0 ≤ y ≤ x, which reflects that only a fraction, y/x, of the Fe(OH)₂ component in ferroan brucite has reacted. Indeed, thermodynamic equilibrium may well be achieved for y < x.

H₂ production rate and brucite oxidation rate. The amount of produced H₂ was used to retrieve the progress of Reaction 2. Magnetite was not used to infer reaction progress because, due to the presence of minor Fe³⁺ in the starting material, part of the magnetite product may form independently of Reaction 2, i.e. without H₂ production (Carlin et al., 2023

Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) Fe^III-substituted brucite: Hydrothermal synthesis from (Mg_0.8Fe^II_0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845

). The amount of H₂ produced in the SP experiments did not induce pressure changes significant enough to retrieve isothermal H₂-production rate laws directly from pressure monitoring. However, the qualitative pressure evolution was used to determine the overall duration of the H₂ production stage, which, in turn, was used to model the reaction kinetics (see Supplementary Information for details). H₂ production kinetics at 378 K could be accurately retrieved from Run SP#6, where gas was regularly sampled (Fig. 2a). In addition, a series of experiments in gold capsules (caps#t5 to #t12) were stopped after different durations and H₂ was analysed in order to further constrain the kinetics of Reaction 2 (Fig. 2b). These experimental data were fitted to a kinetic law (Lasaga, 1998

Lasaga, A.C. (1998) Kinetic Theory in the Earth Sciences. Princeton University Press, Princeton, NJ.

; see Supplementary Information for details) with the reaction rate (r) as:

where with k₀ a kinetic constant, A the Fe(OH)₂ specific surface area, E_a the activation energy, R the gas constant and T the temperature. Q and K are the reaction quotient and the equilibrium constant of Reaction 1, respectively. The standard state is defined here with unit activity for pure minerals and water at any temperature and pressure, as well as unit fugacity for ideal gas at 1 bar of pressure and any temperature. Q was approximated to with P_H2 the H₂ partial pressure at the conditions of the experiment and X_Fe(OH)2 the molar fraction of Fe(OH)₂ in ferroan brucite, by assuming ideal behaviour for H₂ and Fe(OH)₂ in the gas phase and in the brucite solid-solution, respectively. X_Fe(OH)2 was calculated from Equation 2, based on the number of moles of produced H₂ (n_H2). P_H2 was derived from n_H2 considering the amount of H₂ dissolved in the solution as calculated using PHREEQC (Parkhurst and Appelo, 2013

Parkhurst, D.L., Appelo, C.A.J. (2013) Description of Input and Examples for PHREEQC Version 3—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. USGS Techniques and Methods 6–A43, U.S. Geological Survey, Denver, CO. https://pubs.usgs.gov/tm/06/a43/

). K was estimated with the same procedure as Q, by considering that equilibrium H₂ pressure equals the H₂ partial pressure in the gas at the last measurement multiplied by a factor, λ, slightly above 1 (see Supplementary Information for details on this factor). Fitted r₀ values are displayed in Figure 2c as a function of reciprocal temperature. The slope in the linear fit corresponds to an activation energy of 145 ± 1 kJ/mol. The intercept of the fit provides a k₀ × A quotient of 8.97 × 10⁷ mol s⁻¹ g⁻¹.

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Experimental constraints on thermodynamic properties of ferroan brucite. Based on the kinetic law derived in the previous section, 17 experiments reached equilibrium (Table S-2). These experiments were, thus, used to constrain the equilibrium constant (K) of Reaction 1.

As discussed above, a set of K values can be calculated based on n_H2, measured at the end of each experiment. A pair of Δ_fH° and S° values for the Fe(OH)₂ end member was retrieved by least square regression through this set of K values (see Supplementary Information for thermodynamic calculation details). Δ_fH°_Fe(OH)2 and S°_Fe(OH)2 of −581.3 ± 2.9 kJ/mol and 86.4 ± 6.3 J/mol/K were obtained, respectively. These thermodynamic values are only relevant for calculations using the same standard states as those used here, as well as the same assumption of unit activity and fugacity coefficients for ferroan brucite solid solution and H₂ in the gas phase, respectively. They fall in the range of published values for Fe(OH)₂ (Table S-4, Fig. S-3). The Δ_fH°_Fe(OH)2 value is consistent with the value by Ziemniak et al. (1995)

Ziemniak, S.E., Jones, M.E., Combs, K.E.S. (1995) Magnetite solubility and phase stability in alkaline media at elevated temperatures. Journal of Solution Chemistry 24, 837–877. https://doi.org/10.1007/BF00973442

and departs by 1.2 % from Δ_fH°_Fe(OH)2, tabulated in the NIST-JANAF database (Chase, 1998

Chase, M.W. (1998) NIST-JANAF Thermochemical Tables. Fourth Edition, American Chemical Society, Washington, D.C.

). The values of the NIST-JANAF database only differ by 0.1 % from those commonly used for thermodynamic modelling of fluid–rock interactions in ultramafic rocks (McCollom and Bach, 2009

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032

). H₂ production prediction for our experiments is overestimated by more than one order of magnitude with the McCollom and Bach’s (2009)

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032

database (Fig. S-4).

Implications for low-T H₂ production in ultramafic systems. Olivine serpentinisation (e.g., McCollom et al., 2016

McCollom, T.M., Klein, F., Robbins, M., Moskowitz, B., Berquó, T.S., Jöns, N., Bach, W., Templeton, A. (2016) Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine. Geochimica et Cosmochimica Acta 181, 175–200. https://doi.org/10.1016/j.gca.2016.03.002

) and ferroan brucite alteration (Miller et al., 2017

Miller, H.M., Mayhew, L.E., Ellison, E.T., Kelemen, P., Kubo, M., Templeton, A.S. (2017) Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite. Geochimica et Cosmochimica Acta 209, 161–183. https://doi.org/10.1016/j.gca.2017.04.022

; Ellison et al., 2021

Ellison, E.T., Templeton, A.S., Zeigler, S.D., Mayhew, L.E., Kelemen, P.B., Matter, J.M., The Oman Drilling Project Science Party (2021) Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite. Journal of Geophysical Research: Solid Earth 126, e2021JB021981. https://doi.org/10.1029/2021JB021981

) are the two main processes that have been proposed to account for H₂ production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016

McCollom, T.M., Donaldson, C. (2016) Generation of Hydrogen and Methane during Experimental Low-Temperature Reaction of Ultramafic Rocks with Water. Astrobiology 16, 389–406. https://doi.org/10.1089/ast.2015.1382

), a maximum of 0.028 nmol H₂/g olivine/day was proposed for H₂ production by olivine serpentinisation at 363 K (Fig. 3a). In comparison, the rate of H₂ production measured here during ferroan brucite reaction (r₀) is approximately three orders of magnitude higher (Fig. 3a). When weighted by the specific surface area of the powders used in the various experiments, however, the reaction rates are rather well aligned in an Arrhenius plot with an activation energy of ∼81 kJ/mol (Fig. 3b). The difference in grain sizes between olivine used in the experiments depicted in Figure 3 (38–212 μm) and the synthetic ferroan brucite used here (∼50 nm) probably plays a key role in their difference of reactivity.

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The respective contribution of olivine and Fe-brucite alteration to the H₂ production rate in natural systems was evaluated using a numerical approach. A fluid–serpentinised dunite system composed of olivine (grain size of 500 μm; Malvoisin et al., 2017

Malvoisin, B., Brantut, N., Kaczmarek, M.-A. (2017) Control of serpentinisation rate by reaction-induced cracking. Earth and Planetary Science Letters 476, 143–152. https://doi.org/10.1016/j.epsl.2017.07.042

), ferroan brucite (grain size of 50 nm; Malvoisin et al., 2021

Malvoisin, B., Auzende, A.-L., Kelemen, P.B., the Oman Drilling Project Science Party (2021) Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration. Earth and Planetary Science Letters 576, 117212. https://doi.org/10.1016/j.epsl.2021.117212

) and water was modelled with H₂ being only produced by alteration of the latter minerals. A range of escape rates of H₂ (advection and/or diffusion) was defined in order to simulate hydrothermal activity at mid-ocean ridges, water infiltration in an ophiolitic unit or sub-stagnant hydraulic conditions in a deep aquifer. The temperature was set to 363 K, relevant to low-T serpentinisation (Fig. 4; see model details in Supplementary Information).

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The model shows that ferroan brucite is the first mineral to react with a rate that is three orders of magnitude faster than that of olivine, leading to a rapid H₂ production in the first year of the reaction (Fig. 4b). In a closed system, ferroan brucite reaction rapidly stops due to the attainment of thermodynamic equilibrium (Q/K = 1). The adjustment of the thermodynamic parameters of Fe(OH)₂ proposed here (1.8 % and 1.2 % for S°_Fe(OH)2 and Δ_fH°_Fe(OH)2, respectively, compared to the database of McCollom and Bach, 2009

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032

) has a strong impact on the predicted equilibrium H₂ partial pressure and, thus, on the amount of H₂ that is produced. At 363 K, equilibrium is achieved for a H₂ molality of 2.2 × 10⁻⁵ mol/kg, while it is two orders of magnitude higher (5.0 × 10⁻³ mol/kg) with the database of McCollom and Bach (2009)

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032

. At 313 K, a difference of three orders of magnitude for H₂ molality between the two database is predicted. The oxygen fugacity (fO₂) lies on the H₂O/H₂(g) equilibrium with the database of McCollom and Bach (2009)

McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032

and five orders of magnitude above with the thermodynamic data derived here. Interestingly, this latter fO₂ is consistent with the fO₂ measured at the bottom of Holes BA1A, BA1D and BA4A during the Oman Drilling Project (Kelemen et al., 2021

Kelemen, P.B., Leong, J.A., de Obeso, J.C., Matter, J.M., Ellison, E.T., Templeton, A., Nothaft, D.B., Eslami, A., Evans, K., Godard, M., Malvoisin, B., Coggon, J.A., Warsi, N.H., Pézard, P., Choe, S., Teagle, D.A.H., Michibayashi, K., Takazawa, E., Al Sulaimani, Z., The Oman Drilling Project Science Team (2021) Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon. Journal of Geophysical Research: Solid Earth 126, e2021JB022729. https://doi.org/10.1029/2021JB022729

). After ferroan brucite reaction, olivine completely reacts in the model in approximately 3 Myr and ultimately produces 3000 times more H₂ than ferroan brucite (Fig. 4a).

Ferroan brucite can further react if, at the same time, H₂ escapes from the system at a rate exceeding that of H₂ production associated with olivine serpentinisation (Fig. 4c). This corresponds to minimum H₂ escape rates of 10⁻⁵ and 7 × 10⁻⁴ mol H₂/day/g rock at 313 and 363 K, respectively. The estimated maximum escape rate by vertical diffusion is three orders of magnitude lower than this threshold value (see Supplementary Information for details), suggesting that vertical diffusion is not sufficient to drive ferroan brucite reaction. The threshold value is achieved by water renewal at a minimum rate of 7 × 10⁻⁶ and 3 × 10⁻⁵ kg water/day/kg rock at 313 and 363 K, respectively. The average water-to-rock ratio at mid-ocean ridges is ∼1 (Coogan et al., 2019

Coogan, L.A., Seyfried, W.E., Pester, N.J. (2019) Environmental controls on mid-ocean ridge hydrothermal fluxes. Chemical Geology 528, 119285. https://doi.org/10.1016/j.chemgeo.2019.119285

). Considering hydrothermal activity and fluid flow during a minimum of 30,000 yr (Früh-Green et al., 2003

Früh-Green, G.L., Kelley, D.S., Bernasconi, S.M., Karson, J.A., Ludwig, K.A., Butterfield, D.A., Boschi, C., Proskurowski, G. (2003) 30,000 Years of Hydrothermal Activity at the Lost City Vent Field. Science 301, 495–498. https://doi.org/10.1126/science.1085582

), it can be converted into a mean water flux of 10⁻⁷ kg water/day/kg rock. In the Oman ophiolite, ferroan brucite with x = 0.28 can represent ∼50 mol % of the serpentinisation reaction products (Malvoisin et al., 2020

Malvoisin, B., Zhang, C., Müntener, O., Baumgartner, L.P., Kelemen, P.B., Oman Drilling Project Science Party (2020) Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project. Journal of Geophysical Research: Solid Earth 125, e2019JB018877. https://doi.org/10.1029/2019JB018877

). Present day alteration of such ferroan brucite may occur during interaction with rainwater. H₂-rich hyperalkaline fluids are found at depths >50 m (Leong et al., 2023

Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H₂ and CH₄ outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008

), and the recharge rate of the aquifer in Oman is 18 mm/year (Dewandel et al., 2005

Dewandel, B., Lachassagne, P., Boudier, F., Al-Hattali, S., Ladouche, B., Pinault, J.-L., Al-Suleimani, Z. (2005) A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach. Hydrogeology Journal 13, 708–726. https://doi.org/10.1007/s10040-005-0449-2

). Combining these values leads to a mean meteoritic water flux of 3 × 10⁻⁷ kg water/day/kg rock. Both in ophiolites and on the seafloor, mean water flux estimates are, thus, approximately one order of magnitude lower than the minimum flux necessary to drive ferroan brucite reaction. However, fluid flow in ultramafic rocks is concentrated in cracks and microcracks (Dewandel et al., 2005

Dewandel, B., Lachassagne, P., Boudier, F., Al-Hattali, S., Ladouche, B., Pinault, J.-L., Al-Suleimani, Z. (2005) A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach. Hydrogeology Journal 13, 708–726. https://doi.org/10.1007/s10040-005-0449-2

; Corre et al., 2023

Corre, M., Brunet, F., Schwartz, S., Gautheron, C., Agranier, A., Lesimple, S. (2023) Quaternary low-temperature serpentinization and carbonation in the New Caledonia ophiolite. Scientific Reports 13, 19413. https://doi.org/10.1038/s41598-023-46691-y

) and is, thus, expected to be, locally, several orders of magnitude higher than the mean water flux. For example, the highest water-to-rock ratio values reported in abyssal peridotites are above 10⁵ (Snow and Reisberg, 1995

Snow, J.E., Reisberg, L. (1995) Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites. Earth and Planetary Science Letters 133, 411–421. https://doi.org/10.1016/0012-821X(95)00099-X

; Delacour et al., 2008

Delacour, A., Früh-Green, G.L., Frank, M., Gutjahr, M., Kelley, D.S. (2008) Sr- and Nd-isotope geochemistry of the Atlantis Massif (30°N, MAR): Implications for fluid fluxes and lithospheric heterogeneity. Chemical Geology 254, 19–35. https://doi.org/10.1016/j.chemgeo.2008.05.018

), corresponding to water fluxes >10⁻² kg water/day/kg rock compatible with H₂ production associated with ferroan brucite oxidation. The measured maximum H₂ production rate in the Oman ophiolite is of 71,000 mol H₂/yr for a minimal volume of altered rock of 0.05 km³ (Leong et al., 2023

Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H₂ and CH₄ outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008

). This corresponds to a specific flux of 10⁻³ nmol H₂/g rock/day which is consistent with the specific H₂ production rate of 5 × 10⁻³ nmol H₂/g rock/day, estimated at 313 K based on the extrapolation of the data acquired here for ferroan brucite alteration (Fig. 3). Actually, the same extrapolation for serpentinisation of olivine having a grain size of 500 μm (Fig. 3b) yields a much lower production rate of 10⁻⁵ nmol H₂/g rock/day at 313 K. Ferroan brucite oxidation could thus be a main contributor to H₂ production at temperatures below 423 K in ophiolites and on the seafloor. This is consistent with petrographic observations in natural samples showing ferroan brucite oxidation to form magnetite in open systems conditions (Bach et al., 2006

Bach, W., Paulick, H., Garrido, C.J., Ildefonse, B., Meurer, W.P., Humphris, S.E. (2006) Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274). Geophysical Research Letters 33, L13306. https://doi.org/10.1029/2006GL025681

; Jöns et al., 2017

Jöns, N., Kahl, W.-A., Bach, W. (2017) Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography. Lithos 268–271, 274–284. https://doi.org/10.1016/j.lithos.2016.11.014

).

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Acknowledgements

This work is part of a CIFRE PhD (n°2019.1672) funded by ENGIE, and performed in collaboration with Storengy. Analyses have been performed at the Geochemistry & Mineralogy platform of ISTerre (Université Grenoble Alpes, France). The editor, E.H. Oelkers, and two anonymous reviewers are thanked for their insightful comments and careful reading.

Editor: Eric Oelkers

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References

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Supplementary Information

The Supplementary Information includes:

• Characterisation Techniques
• Time Needed to Reach Thermodynamic Equilibrium in Titanium Reactors
• H₂ Production Rate-law
• Retrieval of Δ_fH° and S° of the Fe(OH)₂ End Member: Calculation Method
• Numerical Modelling of H₂ Production During Serpentinised Dunite Alteration
• Tables S-1 to S-4
• Figures S-1 and S-4
• Supplementary Information References

Download the Supplementary Information (PDF)